The present invention is related to a method for molding a resin piston which is used, for example, in a disc brake device of a vehicle, and the resin piston.
Conventionally, a resin piston which is used in a disc brake device of a vehicle is usually produced by compression molding method, for the purpose of increasing packing density of material thereby to secure mechanical strength.
FIGS. 3A to 3D show a conventional production method for forming a resin piston by compression molding. This production method is similar to a resin molding method which is disclosed in Patent Document 1.
A molding die 1 to be used in the compression molding includes a lower die 2, and an upper die 3 capable of moving up and down with respect to the lower die 2.
In a production of the resin piston, as a first step, a resin tablet 5 as thermosetting resin material in a preheated state is charged into a cavity 4 in the lower die 2, as shown in FIG. 3A. Then, the upper die 3 is moved down toward the lower die 2, as shown in FIG. 3B, and a pressure punch 6 which is integrally provided with the upper die 3 is pushed into the cavity 4 thereby to crush the resin tablet 5. As downward movement of the upper die 3 proceeds, a part of the resin tablet 5 in the cavity 4 overflows to a passage 9 which is defined between an upper surface 7 of the lower die and a lower surface 8 of the upper die around an outer circumference of the cavity, as shown in FIG. 3C.
In a situation where the resin tablet 5 is discharged to the passage 9 as shown in FIG. 3C, a gap between the lower surface 8 of the upper die and the upper surface 7 of the lower die is gradually made narrow thereby to restrain a flow of the material, and hence, pressure in the cavity 4 is gradually increased. When the upper die 3 is moved down up to a final pressurizing margin, as shown in FIG. 3D, an internal pressure of the cavity arrives at a setting pressure. In this situation, a sectional area of an opening of the passage 9 is reduced up to a prescribed value, and the resin is restrained from flowing to the passage 9, and hence, the internal pressure of the cavity is maintained. Moreover, in this situation, the passage 9 is not completely closed for realizing discharge of superfluous resin material, and kept communicated with the cavity 4.
In the state as shown in FIG. 3D, a thermosetting process for thermosetting the resin material in the cavity 4 is carried out in the molding die 1 which has been warmed by heating to a predetermined temperature, in advance. Then, the thermosetting process is carried out to such an extent that there occurs no dimensional change after a molded product is taken out from the molding die, and thereafter, the molding die 1 is opened to take out the molded product.
On occasion of thermosetting the resin material in the cavity 4, the heating is usually conducted by heating the molding die 1 from outside to raise the temperature, and therefore, it is difficult to efficiently heat the material inside the mold.
In view of the above, there is proposed, in Patent Document 1, a compression molding method for realizing reduction of time required for thermosetting process of the resin, by adding a high frequency induction heating coil to the molding die.
There is also proposed, in Patent Document 2, a method for molding a resin piston, by charging a preheated resin tablet into a cavity of a molding die, and by compression molding the resin tablet while heating, wherein a mold is closed in such a state that a movable core for forming a concave of the resin piston retreats by a certain distance from a determined position for constituting a final shape of a molded product of a brake piston, the resin is injected and charged into the cavity from a communicating passage which communicates between an outer circumference of the cavity and an injection gate, a gate seal pin is advanced to the communicating passage thereby to seal the gate, and thereafter, the movable core is advanced to the determined position thereby to form the final shape of the molded product.
In the compression molding method disclosed in Patent Document 2, an injection molding method is combined with the compression molding method. By combining the methods in this manner, it is possible to add such advantages of the injection molding method that the resin material can be easily charged, and even a complicated shape can be molded, to the compression molding method.
In case of the conventional compression molding method similar to Patent Document 1, as shown in FIGS. 3A to 3D, the passage 9 for discharging the superfluous resin material is made narrow, and sliding resistance of the resin material is enhanced, while the pressure in the cavity is maintained, thereby to secure density in the mold. However, because the molding die is not completely closed, it is difficult to restrain flow of the resin material, which leads to an increase of waste resin material.
Further, the resin material 11 which has flowed out to the passage 9 is left as a burr, on an outer circumferential surface of the resin piston obtained as the molded product, and an additional working step for removing this burr is required. Therefore, there is also such a problem that an increase of cost is incurred.
On the other hand, in the compression molding method disclosed in Patent Document 2, the resin material is charged into the cavity from the injection gate, and therefore, such a large burr as occurring in case of the conventional compression molding method does not occur on the outer circumference of the resin piston as the molded product.
However, because the injection gate is communicated with the outer circumference of the cavity, a mark of the injection molding gate is left on the outer circumferential surface of the resin piston which is the molded product. Accordingly, an additional working step for removing the gate mark such as cylindrical grinding is necessary, and there is also such a problem that the additional working step incurs an increase of cost.
[Patent Document 1] Japanese Patent Publication No. 2005-035077
[Patent Document 2] Japanese Patent Publication No. H08-300430